Production of fibre reinforced ceramic composite

ABSTRACT

A process for the production of a fibre-reinforced ceramic composite by forming a precursor structure comprising a matrix of a composition comprising particulate ceramic material, liquid diluent and organic binder and fibres within the matrix, in which the fibres are formed from a composition comprising particulate ceramic material, liquid diluent and organic binder, and heating the precursor structure in order to evaporate the liquid diluent, decompose the organic binder and sinter the particles of ceramic material in both the matrix and the fibres. Also, a precursor structure as described. The ceramic composite may have a density of 95% or more of the maximum theoretical density.

This is a division of application Ser. No. 07/441,606, filed Nov. 27,1989, now U.S. Pat. No. 5,053,175.

This invention relates to the production of a fibre-reinforced ceramiccomposite and to a precursor structure from which such afibre-reinfocred ceramic composite may be produced.

Fibre-reinforced ceramic composites comprising a matrix of sinteredparticulate material and fibres of a sintered ceramic materialdistributed through the matrix are a promising class of structuralmaterial for use in applications where high strength, high stiffness,low thermal expansion, and high thermal stability are desired, andparticularly where high toughness is desired, and many methods have beenproposed for the production of such fibre-reinforced ceramic composites.However, the previously proposed methods suffer from disadvantages.

Ideally it would be of considerable benefit if the conventional methodsby which sintered ceramic structures are produced could be modifiedreadily in order to incorporate fibres into the structures. Suchsintered ceramic structures are conventionally produced by forming ahomogeneous composition of a particulate ceramic material, a liquiddiluent, and an organic binder, e.g. an organic polymer, in solution orin suspension in the liquid diluent, The composition is formed into adesired shape, for example by compressing the composition in a mould, orby extruding or by injection moulding the composition, the compositionis heated to remove the liquid diluent and to burn off the organicbinder, and the composition is then heated at a higher temperature inorder to sinter together the particles of ceramic material.

It is possible to modify such a conventional production process, forexample, by including fibres of a sintered ceramic material in thecomposition from which the ceramic structure is produced.

However, such a modified production process does suffer fromdisadvantages. Thus, where a homogeneous composition of particulateceramic material, liquid diluent, organic binder, and fibres of asintered ceramic material is formed, for example by use of a bladedmixer or by calendering the composition under conditions of high shear,the fibres may be mechanically damaged. Even where the fibres are notmechanically damaged during the processing, e.g. when a composition isproduced by forming layers of fibres and layers of a matrix composition,the presence of fibres in the composition still has a deleterious effecton the processing of the composition and on the properties of theceramic structure which is produced. Thus, as the composition isprocessed by heating to elevated temperature the composition tends tocontract as the liquid diluent is removed, as the organic binder isburned off, and as the particles of ceramic material are sintered, andthe composition also tends to contract as it is cooled from the elevatedprocessing temperature in the final stages of the process. However,there is generally a substantial mismatch between the thermalcharacteristics of the fibres and rest of the composition as the fibresare already sintered and they do not contract during heating of thecomposition, and furthermore the fibres in the composition tend toresist the contraction of the composition. The overall result is thatcracks form in the matrix of the ceramic composite produced from thefibre-containing composition, the density of the ceramic composite isrelatively low, in comparison with the maximum theoretical density, e.g.the density may be as low as 75% and rarely as high as 85% of themaximum theoretical density, and the fibres in the composite tend not tobe bonded to the matrix of the composite with the result that themechanical properties of the composite are adversely affected. Whilst itis possible to overcome to some extent this adverse effect on themechanical properties of the composite by processing thefibre-containing composition under an applied pressure this is not onlyinconvenient but use of pressure also places a limitation of thecomplexity of the shape of the composite which can be produced. Also theresultant composite still may not have mechanical properties, e.g.tensile properties and toughness, or density which are as great as mayhave been desired.

Other methods have been proposed for producing fibre-reinforced ceramiccomposites. For example, a three dimensional structure formed of fibresof a ceramic material, e.g. a structure formed of a mat or of a stack ofa plurality of mats of fibres of a ceramic material, may be impregnatedwith a composition of particulate ceramic material, liquid diluent, andorganic binder, and the thus impregnated structure may be furtherprocessed by heating as hereinbefore described. However, thisimpregnation process tends to result in production of a composite whichhas the disadvantageous features hereinbefore referred to as a result ofthe tendency of the fibres not to contract and to resist contraction ofthe composition, and furthermore in effecting the impregnation thefibres in the structure tend to filter the particles of ceramic materialand it is thus difficult to effect a homogeneous impregnation of thestructure with the result that the composite which is produced also hasan inhomogeneous composition and variable mechanical properties.

Fibre-reinforced ceramic composites can be produced by a meltinfiltration technique in which a structure of a ceramic fibrousmaterial is impregnated with a melt of a ceramic material. However, veryhigh temperatures must necessarily be used and also some ceramicmaterials sublime rather than melt. Furthermore as a result of thegenerally high viscosity of melts of ceramic materials the rate ofinfiltration of the melt into the fibrous structure may be very low andit may be difficult to infiltrate the whole structure homogeneously, andalso the fibres of ceramic material may be damaged at the highprocessing temperature involved in the use of melts of ceramicmaterials.

A further process by which fibre-reinforced ceramic composites may beproduced is the so-called chemical vapour infiltration process in whicha structure formed of fibres of a ceramic material is infiltrated withvapour of a material which can be decomposed to form the matrix ofceramic material in the composite. The process may be operated atrelatively low temperature, although generally at a temperature of theorder of several hundreds of degrees centigrade, with the result thatthe damage to the fibres of the ceramic material which may be associatedwith the melt infiltration process is at least to some extent mitigated.An example of a material which in vapour form may be infiltrated into astructure formed of fibres of a ceramic material ismethyltrichlorosilane which may be decomposed to form silicon carbide byheating to a temperature which may be less then 1200° C. For example, asilicon carbide fibrous structure may be infiltrated with the vapour ofmethyl trichlorosilane and the latter may be thermally decomposed in thefibrous structure to form silicon carbide, the product which is producedin the process being a composite comprising a matrix of silicon carbidereinforced by fibres of silicon carbide. Although the chemical vapourinfiltration process does itself overcome some of the disadvantages ofprocesses previously described, for example, the damage to the fibres ofceramic material associated with the melt infiltration process, it isitself a very time-consuming process. Indeed, the processing timeinvolved in the production of a fibre-reinforced composite may be aslong as several weeks.

The present invention relates to a process for the production of afibre-reinforced ceramic composite, that is a composite which comprisesa matrix of a ceramic material having fibres of a ceramic materialdispersed therein as a reinforcement. The process of the invention issimple to operate in that it is a modification of a conventional processas hereinbefore described in which a structure of sintered particulateceramic material is produced from a composition comprising a particulateceramic material, a liquid diluent and an organic binder. However, themodification of the present invention does not suffer from thedisadvantages hereinbefore described, particularly the contractionproblems associated with the production of a fibre-reinforced ceramiccomposite from a structure comprising a matrix of particulate ceramicmaterial, liquid diluent and organic binder admixed with fibres of asintered ceramic material, indeed, the ceramic composite which isproduced may have a density 95% or more of the maximum theoreticaldensity. The present invention also provides a precursor structure fromwhich a fibre-reinforced ceramic composite structure may be produced.

The present invention provides a process for the production of afibre-reinforced ceramic composite by forming a precursor structurecomprising a matrix of a composition comprising particulate ceramicmaterial, liquid diluent and organic binder and fibres within thematrix, in which the fibres are formed from a composition comprisingparticulate ceramic material, liquid diluent and organic binder, andheating the precursor structure in order to evaporate the liquiddiluent, decompose the organic binder and sinter the particles ofceramic material in both the matrix and the fibres.

The invention also provides a precursor structure from which such afibre-reinforced ceramic composite may be produced and which comprises amatrix of a composition comprising particulate ceramic material, liquiddiluent and an organic binder and fibres within the matrix which areformed from a composition comprising particulate ceramic material,liquid diluent and organic binder.

In the process by which the fibre-reinforced ceramic composite isproduced by heating the precursor structure both the matrix and thefibres contract due in part to loss of liquid diluent and organicbinder, and also to sintering of the particles of ceramic material, bothin the matrix and in the fibre, with the result that in thefibre-reinforced ceramic composite produced by the process of theinvention there is a reduced tendency for cracks to form in thecomposite and a reduced tendency for the fibres not to be bonded to thematrix when compared with the hitherto described process in which such acomposite is produced from a precursor structure comprising a matrix ofparticulate ceramic material, liquid diluent and organic binder inadmixture with fibres of ceramic material which are already sintered andin which there is substantial differential shrinking between the matrixand the fibres.

In the precursor structure from which the fibre-reinforced ceramiccomposite is produced the particulate ceramic material in thecomposition from which the matrix is formed may be the same as ordifferent from the particulate ceramic material in the composition fromwhich the fibres are formed. Similarly, the liquid diluent, and theorganic binder, in the composition from Which the matrix is formed maybe the same as or different from the liquid diluent, and the organicbinder, in the composition from which the fibres are formed.

The relative proportions of particulate ceramic material, liquiddiluent, and organic binder in the compositions from which the matrixand the fibres are produced may be the same or different. However,during heating of the precursor structure to form the fibre reinforcedceramic composite both the matrix and the fibres contract, due in partto loss of liquid diluent and to loss of organic binder from therespective compositions, and also to the sintering of the particles ofceramic material, and it is preferred that the extent of contraction ofthe matrix and of the fibres caused by loss of liquid diluent anddecomposition of organic binder should be substantially the same or atleast similar, and for this reason it is preferred that the compositionsfrom which the matrix and the fibres are produced should besubstantially the same in respect of the relative proportions of thecomponents therein.

Any particulate ceramic material may be used in the process of theinvention. Thus, the particulate ceramic material may be an oxide or amixture of oxides of a metallic or of a non-metallic element, forexample, an oxide of aluminium, calcium, magnesium, silicon, chromium,hafnium, molybdenum, thorium, uranium, titanium or zirconium. Theceramic material may be a carbide of, for example, boron, chromium,hafnium, molybdenum, niobium, tantalum, thorium, titanium, tungsten,uranium, zirconium or vanadium. The ceramic material may be siliconcarbide. The ceramic material may be a boride or a nitride, for example,a boride or a nitride of one or more of the elements hereinbeforereferred to.

The ceramic material is a material which may be heated to an elevatedtemperature, e.g. to a temperature in excess of 1000° C., to cause theparticles of the material to sinter together. Within the scope of theterm particulate ceramic material there is also included those metalswhich when in a powdered form can be sintered or fused together byapplication of heat, that is those metals which are susceptible ofprocessing by the technique of powder metallurgy. Suitable metalsinclude aluminium and its alloys, copper and its alloys and nickel andits alloys.

The particulate ceramic material may be a mixture of particles, forexample, comprising a mixture of a particulate metal or metals and/or aparticulate ceramic non-metallic material or material.

The particles of ceramic material in the compositions from which thematrix and the fibres are produced may have any convenient size,although it is preferred that they are of relatively small size,particularly those in the composition from which the fibre is producedas the fibre itself may be of relatively small diameter and theparticles of ceramic material in the composition from which the fibre isproduced should have a maximum dimension substantially smaller than thediameter of the fibre.

It is preferred that the particles of ceramic material are of relativelysmall size, for example a size of less than 5 microns. Particles havinga size of less than 1 micron and even less than 0.2 micron are morepreferred as the use of such particles enables sintering of theparticles of ceramic material to be effected at lower temperatures andat faster rates than would be the case with larger size particles. Theparticulate ceramic materials may have a mono-modal particle sizedistribution, that is, the particles may be all of substantially thesame size, or the particle size distribution may be multi modal, thatis, the particles may comprise a plurality of sizes.

In order that the particulate ceramic material in both the matrix and inthe fibre of the precursor structure formed in the process of theinvention should sinter in a similar manner, for example at a similarrate at a given temperature, and in order that the matrix and the fibrein the structure should contract at a similar rate on sintering of theparticles of ceramic material, it is preferred that the particulateceramic material in the compositions from which the matrix and the fibreare produced is the same and that the particles have a substantiallysimilar size and size distribution. A similar rate of sintering of theparticles of ceramic material in the matrix and in the fibre and asimilar rate of contraction of the matrix and the fibre on sintering ofthe particles has a beneficial effect on the properties of thefibre-reinforced ceramic composite produced in the process of theinvention, particularly on the density of the composite.

The liquid diluent may be an organic liquid or it may be an aqueousliquid, e.g. water or a solution of water and an organic liquid. Thenature of the liquid diluent will be determined at least in part by thenature of the organic binder in the compositions from which the matrixand fibre are produced. It is desirable that the organic binder besoluble in the liquid diluent and the liquid diluent will be selectedaccordingly. The liquid diluent may be an alcohol, especially a loweralcohol, e.g. methanol or ethanol, but for reasons of non-toxicity andnon-flammability, it is preferably water.

The function of the organic binder in the compositions from which thematrix and the fibres are produced is to bind together the particles ofceramic material in the matrix and in the fibre in the precursorstructure produced in the process prior to the particles being sinteredin the later heating stages of the process. The binder Will generally bean organic polymeric material and it is preferably soluble in the liquiddiluent as use of such a soluble polymeric material assists in thesuccessful production of a fibre.

Suitable water soluble organic polymeric materials for use as the binderinclude

(a) cellulose ethers, for example hydroxypropyl methyl cellulose,

(b) amide-substituted polymers, for example a polymer or copolymer ofacrylamide,

(c) polyalkylene oxide derivatives which may be, for example apolyalkylene oxide (alternatively described as a polyalkylene glycol)for example polyalkylene glycols of molecular weight above about 10,000,and

(d) a hydrolysed vinyl acetate polymer or copolymer.

The polymer may be a copolymer of vinyl acetate and a monomercopolymerisable therewith, but it is preferably a hydrolysed poly(vinylacetate). In order to aid solubility in water the degree of hydrolysisof the vinyl acetate (co)polymer will generally be at least 50%,preferably in the range 70% to 90%.

As the liquid medium and the organic binder must be removed from thematrix and from the fibre in the heating stages of the process it ispreferred, in order to avoid excessive shrinkage, that the amounts ofliquid medium and organic binder in the compositions from which thematrix and fibre are produced are not excessive, and in particular it ispreferred that the compositions contain a high proportion of particulateceramic material. The compositions preferably comprises greater than 50%by weight of particulate ceramic material, more preferably at least 70%by weight. The compositions may contain as much as 95% by weight ofparticulate ceramic material.

The proportion of liquid medium in the compositions will generally begreater than 5% by weight but will generally be not more than 25% byweight.

The proportion of organic binder in the compositions, particularly wherethe binder is an organic polymeric material, will be chosen in part togive to the composition a consistency suitable for shaping, particularlyfor shaping into a fibre. The compositions will generally contain atleast 3% by weight of organic binder but generally not more than 20% byweight.

In order that the particulate ceramic material in both the matrix and inthe fibre of the precursor structure formed in the process of theinvention should sinter in a similar manner, for example at a similarrate at a given temperature, and in order that the matrix and the fibrein the structure should contract at a similar rate on removal of liquidmedium and organic binder therefrom and on sintering of the particles ofceramic material, it is preferred that the proportions of particulateceramic material, of liquid medium, and of organic binder in thecompositions from which the matrix and the fibre are produced should besubstantially the same.

The components of the compositions from which matrix and the fibre areproduced may be mixed in a similar manner, for example, by mixing in abladed mixer. However, it is preferred that the components of thecompositions are homogeneously mixed and to this end mixing underconditions of high shear is preferred, as in a screw extruder. Apreferred form of high shear mixing is that which may be effected on atwin roll mill the rolls of which may be operated at the same ordifferent peripheral speeds. The compositions may be passed repeatedlythrough the nip between rolls of the mill, which nip may beprogressively decreased in size. The nip between the rolls of the millmay be decreased to a size as low as 0.1 mm with the result that veryhigh shear may be applied to the compositions which assists in breakingdown aggregates of particulate ceramic material which may be present inthe compositions and in the production of a homogeneously mixedcomposition.

The fibre which forms part of the precursor structure of the inventionmay be formed by extruding a composition through a suitable orifice. Thecomposition will have a consistency such that when the composition isextruded in fibrous form the fibre is able to maintain its integrity.The fibre may be extruded as a continuous filament, or it may be choppedinto the form of relatively short fibres. The fibre may be convertedinto the form of a mat, for example, by weaving of the fibre.

The fibre may be of any suitable diameter. It will generally have adiameter of at least 50 microns, and it may have a diameter of as muchas 500 microns or even 1 mm or greater. No particular limitation needsto be placed on the fibre diameter.

In order to assist in maintaining the integrity of the fibre duringformation of the precursor structure, and during heating of thestructure to produce the fibre-reinforced ceramic composite, the fibremay be coated with a material which is resistant to the elevatedtemperatures encountered in the heating stages of the process, forexample, which is resistant to elevated temperatures of 1000° C. ormore. A suitable coating for the fibre is carbon, which may be appliedto the surface of the fibre by evaporation or by contacting the fibrewith a dispersion of carbon in a liquid medium. The fibre may be coatedwith the decomposible precursor of a material which is resistant toelevated temperatures, for example, with a decomposible precursor of arefractory boride.

Although the precursor structure may be formed by a variety of differenttechniques certain techniques by which fibre-reinforced structures maybe formed in other arts e.g. in the fibre reinforced plastics art, mayprove to be unsuitable. Thus mixing of a matrix-forming composition withfibres in a bladed mixer, and particularly mixing of a matrix-formingcomposition with fibres under conditions of high shear, e.g. in anextruder or on a twin roll mill, may tend to destroy the integrity ofthe fibres with the result that the desired fibrous reinforcement of theceramic composite produced in the process may not be achieved.

The precursor structure is desirably formed by methods which do notresult in destruction of the integrity of the fibres. For example, thecomposition from which the matrix is produced may be in the form of asheet, and fibres, which may be in the form of relatively short choppedfibres or in the form of a mat which may be woven or unwoven, or inother forms, may be placed on the sheet and optionally pressed into thesheet.

A precursor structure may be built up by forming a plurality ofalternating layers of a composition from which the matrix is producedand a layer of fibres and pressing the structure.

The precursor structure may be formed by other methods and the processof the invention is not limited to use of a particular method of formingthe precursor structure. For example, the precursor structure may beformed by pressing a mass of fibres which has been coated with thecomposition from which the matrix is produced.

In the subsequent stages of the process of the invention the precursorstructure is heated in order to evaporate the liquid diluent, decomposethe organic binder, and sinter the particles of ceramic material in boththe matrix and the fibre.

The precursor structure need be heated at only a relatively lowtemperature in order to evaporate the liquid medium, a temperature of upto 100° C. or possibly slightly higher generally sufficing, although thetemperature to be used will depend to some extent on the nature of theliquid medium.

Similarly, the nature of the organic binder will determine at least tosome extent the temperature to which the precursor structure should beheated in order to decompose the binder and remove the binder from thestructure. In general a temperature of up to 500° C. may suffice,although a higher temperature may be used e.g. a temperature of up to750° C.

Similarly, the temperature at which sintering of the particles ofceramic material in the matrix and in the fibre may be effected willalso depend at least to some extent on the nature of the ceramicmaterial and on the form of the ceramic material, e.g. the particle sizeand the particle size distribution. The temperature at which sinteringmay be effected will generally be at least 1000° C. and it may even beup to a temperature of 1000° C. or greater.

Although in the aforementioned description specific temperatures havebeen referred to at which to remove the liquid diluent and decompose theorganic binder, and sinter the particles of ceramic material, theseprocess steps may be effected by heating the precursor structuregradually to progressively higher temperatures, with the temperaturepossibly being maintained at a particular temperature or temperaturesfor a specific period of time during the heating to progressively highertemperatures.

In order to avoid undesirable oxidation it may be necessary to effectsome at least of the heating in a non-oxidising atmosphere, e.g. in anatmosphere of an inert gas. Use of such an atmosphere may beparticularly desirable at the higher temperatures.

The invention is illustrated by the following examples in which allparts are expressed as parts by weight.

EXAMPLE 1

Production of fibre. A composition of 49.5 parts of silicon carbidepowder having a particle size of 0.2 micron, 0.5 parts of boron powder,4.5 parts of hydrolysed polyvinyl acetate having a degree of hydrolysisof 80% and 9 parts of water were mixed on a twin roll mill and formedinto a band on the mill. The band was repeatedly removed from the milland re-inserted through the nip between the rolls of the mill in orderto mix the components of the composition thoroughly. The composition wasthen charged to a screw extruder and extruded in a fibrous form througha 300 micron diameter die on the extruder.

Production of matrix. A composition which was the same as that describedabove, except that the composition contained 5 parts of hydrolysedpolyvinyl acetate, was mixed on a twin roll mill following the abovedescribed procedure and the resultant sheet was removed from the mill.The sheet, which had a thickness of 0.2 mm, was cut into two equal sizedparts.

Production of precursor structure. Fibres produced as described abovewere chopped to a length of approximately 80 mm and placed on thesurface of one of the sheets and the other sheet was then placed on topof the fibres and the thus formed structure was pressed under an appliedpressure of 4 tons.

The production of ceramic composite. The precursor structure was thenheated at 80° C. for 12 hrs, and thereafter the temperature was raisedat 1° C. per minute to 700° C. and the structure was heated at 700° C.for 1 hr in an atmosphere of argon. The temperature was then raised at arate of 15° C. per minute to 2050° C. and heating at 2040° C. wascontinued for 1/2 hr. The thus formed fibre reinforced ceramic compositewas then cooled to ambient temperature. The density of the composite was98% of the maximum theoretical density and examination by optical andelectron microscopy showed that the composite was free of cracks andthat the fibre integrity had been maintained.

COMPARATIVE EXAMPLE 1

Production of fibre. The fibre production process as described inExample 1 above was repeated except that the fibre which was producedwas additionally heated to 2040° C. at a rate of temperature increase of15° C. per minute and the temperature was held at 2040° C. for 30minutes. The fibre of sintered silicon carbide which was produced wasthen coated with a thin layer of carbon by evaporation.

Production of matrix. Two 0.2 mm thick sheets were produced followingthe procedure described in Example 1 above.

Production of precursor structure. A precursor structure was producedfollowing the procedure described in Example 1 above.

Production of ceramic composite. The precursor structure was heatedfollowing the procedure described in Example 1 above. However, the finaldensity of the composite was only 81% of the maximum theoretical densityand optical examination of the composite indicated that the compositecontained substantial porosity mainly present as large cracks transverseto the direction of the fibres.

COMPARATIVE EXAMPLE 1a

The procedure of Comparative Example 1 above was repeated except thatthe precursor structure was produced by pressing the fibres into asurface of one of the matrix sheets.

The final density of the resultant ceramic composite was 87% of thetheoretical maximum density and the composite contained large crackstransverse to the direction of the fibres. Furthermore, the sheet was nolonger planar and the face of the sheet containing the fibres wascurved.

EXAMPLE 2

Production of fibre. The procedure of Example 1 above was followed toproduce a fibre except that the fibre was produced from a composition of50 parts of titanium diboride having a mean particle size of 1 micronand 11 parts of a 20:80 w:w solution of hydroxy propyl methyl celluloseand water, the fibre was extruded through a 500 micron diameter die, andthe fibre after drying was coated with a layer of carbon by dipping in acarbon slurry.

Production of matrix. Two 0.2 mm thick sheets were produced followingthe procedure of Example 1 above except that the sheets were producedfrom a titanium diboride-containing composition as described above.

Production of precursor structure. The procedure described in Example 1above was followed to produce a precursor structure from the titaniumdiboride-containing fibres and matrix produced as described above.

Production of ceramic composite. A ceramic composite was produced fromthe precursor structure following the hearing procedure described inExample 1. The density of the composite was 94% of the theoreticalmaximum density. The integrity of the fibres had been maintained in thecomposite and the composite was free from cracks.

COMPARATIVE EXAMPLE 2

The procedure of Example 2 was followed to produce a ceramic compositeexcept that the composite was produced from a precursor structure inwhich the fibres which were present were titanium diboride fibresproduced as described above which, prior to being coated with carbon,had been heated to 2040° C. at a rate of temperature increase of 15° C.per minute and held at this temperature for 30 minutes in order tosinter the titanium diboride particles in the fibre. The fibre had adensity of 93% of the theoretical maximum. The ceramic composite whichwas produced had a density of only 81% of the theoretical maximumdensity and contained large cracks transverse to the direction of thefibres.

EXAMPLE 3

Production of fibre. The procedure of Example 1 above was followed toproduce a fibre except that the fibre was produced from a composition of50 parts of titanium carbide having a mean particle size of 1.45 micron,5 parts of 80% hydrolysed polyvinyl acetate, 6 parts of water, and thefibre after drying was coated with a layer of carbon by dipping in acarbon slurry.

Production of matrix. Two 0.2 mm thick sheets were produced followingthe procedure of Example 1 above except that the sheets were producedfrom a titanium carbide-containing composition as described above.

Production of precursor structure. The procedure described in Example 1above was followed to produce a precursor structure from the titaniumcarbide-containing fibres and matrix produced as described above.

Production of ceramic composite. A ceramic composite was produced fromthe precursor structure following the heating procedure described inExample 1. The density of the composite was 96% of the theoreticalmaximum density. The integrity of the fibres had been maintained in thecomposite and the composite was free from cracks.

COMPARATIVE EXAMPLE 3

The procedure of Example 3 was followed to produce a ceramic compositeexcept that the composite was produced from a precursor structure inwhich the fibres which were present were titanium carbide fibresproduced as described above which, prior to being coated with carbon,had been heated to 2040° C. at a rate of temperature increase of 15° C.per minute and held at this temperature for 30 minutes in order tosinter the titanium carbide particles in the fibre. The fibre had adensity of 94% of the theoretical maximum. The ceramic composite whichwas produced had a density of only 83% of the theoretical maximumdensity and contained large cracks transverse to the direction of thefibres.

EXAMPLE 4

Production of fibre. The procedure of Example 1 above was followed toproduce a fibre except that the fibre was produced from a composition of50 parts of titanium dioxide having a mean particle size of 0.2 micron,5 parts of 80% hydrolysed polyvinyl acetate, and 6 parts of water, thefibre was extruded through a 200 micron diameter die, and the fibreafter drying was coated with a layer of boron nitride by dipping in aboron nitride slurry.

Production of matrix. Two 0.2 mm thick sheets were produced followingthe procedure of Example 1 above except that the sheets were producedfrom a titanium dioxide-containing composition as described above.

Production of precursor structure. The procedure described in Example 1above was followed to produce a precursor structure from the titaniumdioxide-containing fibres and matrix produced as described above.

Production of ceramic composite. A ceramic composite was produced fromthe precursor structure following the heating procedure described inExample 1 except that the maximum temperature was 1200° C. The densityof the composite was 98% of the theoretical maximum density. Theintegrity of the fibres had been maintained in the composite and thecomposite was free from cracks.

COMPARATIVE EXAMPLE 4

The procedure of Example 4 was followed to produce a ceramic compositeexcept that the composite was produced from a precursor structure inwhich the fibres which were present were titanium dioxide fibresproduced as described above which, prior to being coated with boronnitride, had been heated to 1200° C. at a rate of temperature increaseof 15° C. per minute and held at this temperature for 30 minutes inorder to sinter the titanium dioxide particles in the fibre. The fibrehad a density of 99% of the theoretical maximum. The ceramic compositewhich was produced was found to be broken into several pieces.

EXAMPLE 5

Production of fibre. The procedure of Example 1 above was followed toproduce a fibre except that the fibre was produced from a composition of50 parts of zirconium dioxide powder, 4 parts of 80% hydrolysedpolyvinyl acetate and 6 parts of water, the fibre was extruded through a200 micron diameter die, and the fibre after drying was coated with alayer of boron nitride by dipping in a boron nitride slurry.

Production of matrix. Two 0.2 mm thick sheets were produced followingthe procedure of Example 1 above except that the sheets were producedfrom a zirconium dioxide-containing composition as described above.

Production of precursor structure. The procedure described in Example 1above was followed to produce a precursor structure from the zirconiumdioxide-containing fibres and matrix produced as described above.

Production of ceramic composite. A ceramic composite was produced fromthe precursor structure following the heating procedure described inExample 1 except that the maximum temperature was 1450° C. The densityof the composite was 99.5% of the theoretical maximum density. Theintegrity of the fibres had been maintained in the composite and thecomposite was free from cracks.

COMPARATIVE EXAMPLE 5

The procedure of Example 5 was followed to produce a ceramic compositeexcept that the composite was produced from a precursor structure inwhich the fibres which were present were zirconium dioxide fibresproduced as described above which, prior to being coated with boronnitride, had been heated at a rate of temperature increase of 15° C. perminute to 1450° C. and held at this temperature for 30 minutes in orderto sinter the zirconium dioxide particles in the fibre. The fibre had adensity of 99.8% of the theoretical. The ceramic composite which wasproduced had a density of only 81.4% of the theoretical maximum densityand contained large cracks transverse to the direction of the fibres.

EXAMPLE 6

Production of fibre. The procedure of Example 5 was followed to producea boron nitride-coated zirconium dioxide containing fibre.

Production of matrix. Two 0.2 mm thick sheets were produced followingthe procedure of Example 1 above except that the sheets were producedfrom a composition of 50 parts of aluminium oxide powder, 5 parts of 80%hydrolysed polyvinyl acetate, and 7 parts of water.

Production of precursor structure. The procedure described in Example 1above was followed to produce a precursor structure from the fibres andmatrix produced as described above.

Production of ceramic composite. A ceramic composite was produced fromthe precursor structure following the heating procedure described inExample 1 except that the maximum temperature was 1450° C. The densityof the composite was 99.2% of the theoretical maximum density. Theintegrity of the fibres had been maintained in the composite and thecomposite was free from cracks.

COMPARATIVE EXAMPLE 6

The procedure of Example 6 was followed to produce a ceramic compositeexcept that the composite was produced from a precursor structure inwhich the fibres which were present were sintered zirconium dioxidefibres produced following the procedure of comparative example 5. Theceramic composite which was produced has a density of only 81% of thetheoretical maximum density and contained large cracks transverse to thedirection of the fibres.

We claim:
 1. A precursor structure from which a fibre-reinforced ceramiccomposite may be produced and which comprises a matrix of a compositioncomprising particulate ceramic material, liquid diluent and an organicbinder, and fibres within the matrix which comprise a composition ofparticulate ceramic material, liquid diluent, and organic binder.
 2. Aprecursor structure as claimed in claim 1 in which the particulateceramic material in the composition of the matrix is the same as theparticulate ceramic material in the composition of the fibres.
 3. Aprecursor structure as claimed in claim 1 in which the extent ofcontraction of the matrix and of the fibres caused by loss of liquiddiluent and decomposition of organic binder on heating the precursorstructure to form a ceramic composite is substantially the same.
 4. Aprecursor structure as claimed in claim 3 in which the proportions ofthe components in the compositions from which the matrix and the fibreare produced are substantially the same.
 5. A precursor structure asclaimed in claim 1 in which the particulate ceramic material is selectedfrom the group consisting of silicon carbide, titanium carbide, titaniumdiboride, titanium dioxide and zirconium dioxide.
 6. A precursorstructure as claimed in claim 1 in which the particle size of theceramic material is less than 5 microns.
 7. A precursor structure asclaimed in claim 1 in which the organic binder is an organic polymericmaterial.
 8. A precursor structure as claimed in claim 1 in which theorganic polymer material comprises a hydrolysed vinyl acetate polymer orcopolymer.
 9. A precursor structure as claimed in claim 1 in which thecompositions from which the matrix and the fibre are produced compriseat least 50% by weight of particulate ceramic material.
 10. A precursorstructure as claimed in claim 1 in which the compositions from which thematrix and the fibre are produced comprise greater than 5% by weight ofliquid medium.
 11. A precursor structure as claimed in claim 1 in whichthe compositions from which the matrix and the fibre are producedcomprise at least 3% by weight or organic binder.
 12. A precursorstructure as claimed in claim 1 in which the fibre has a diameter of atleast 50 microns.
 13. A precursor structure as claimed in claim 1 inwhich the fibre is coated with a material which is resistant to theelevated temperature encountered when the precursor structure is heatedto form a ceramic composite.
 14. A precursor structure as claimed inclaim 1 in which the resistant material is carbon.
 15. A precursorstructure as claimed in claim 1 in which the precursor structurecomprises a matrix in the form of a sheet having fibres pressed into thesheet.
 16. A precursor structure as claimed in claim 1 in which theprecursor structure comprises alternating layers of a composition fromwhich the matrix is produced and layers of fibres.